import java.util.StringTokenizer;
import java.util.Stack;

          public class MyDC
   {       private final char ADD = '+';
             /** constant for addition symbol */
           /** constant for subtraction symbol */
           private final char SUBTRACT = '-';
           /** constant for multiplication symbol */
           private final char MULTIPLY = '*';
           /** constant for division symbol */
           private final char DIVIDE = '/';
           /** the stack */
           private Stack<Integer> stack;

           public MyDC() {
             stack = new Stack<Integer>();
           }

           public int evaluate (String expr)
           {
             int op1,op2,result = 0;
             String token;
             StringTokenizer tokenizer = new StringTokenizer (expr);

             while (tokenizer.hasMoreTokens())
             {
               token = tokenizer.nextToken();

               //如果是运算符调用isOperator
              if (isOperator(token)){
                   op2 =(stack.pop()).intValue();
                   op1 =(stack.pop()).intValue();
                   result = evalSingleOp(token.charAt(0),op1,op2);
                   stack.push(result);
                 //从栈中弹出操作数2
                 //从栈中弹出操作数1
                 //根据运算符和两个操作数调用evalSingleOp计算result;
                 //计算result入栈;
             }
               else
                 stack.push(Integer.parseInt(token));
                 //如果是操作数
                 //操作数入栈;
             }

             return result;
             }

           private boolean isOperator (String token)
           {
             return ( token.equals("+") || token.equals("-") ||
                      token.equals("*") || token.equals("/") );
           }

           private int evalSingleOp (char operation, int op1, int op2)
           {
             int result = 0;

             switch (operation)
             {
               case ADD:
                 result = op1 + op2;
                 break;
               case SUBTRACT:
                 result = op1 - op2;
                 break;
               case MULTIPLY:
                 result = op1 * op2;
                 break;
               case DIVIDE:
                 result = op1 / op2;
             }

             return result;
          }
         }
